Mobile Communication System, Base Station Device and Frequency Allocation Method for Mobile Communication System

ABSTRACT

In a base station device  12  of an OFDMA mobile communication system, a frequency band storage unit  22  stores multiple frequency bands allocatable to mobile station devices, respectively, in association with predetermined conditions related to communication quality. A communication quality acquisition unit  37  acquires communication quality in communication with each of the mobile station devices. A frequency allocator  24  selects any one of the frequency bands from the frequency band storage unit  22  on the basis of the communication quality acquired by the communication quality acquisition unit  37 , and notifies the mobile station device of channel information indicating the selected frequency band. Then, the mobile station device communicates with the base station device  12  in the frequency band indicated by the channel information notified by the frequency allocator  24  in the base station device  12.

TECHNICAL FIELD

The present invention relates to a mobile communication system, a basestation device and a frequency allocation method for the mobilecommunication system. In particular, the present invention relates to amobile communication system for performing communication by orthogonalfrequency division multiplexing, a base station device and a frequencyallocation method for the mobile communication system.

BACKGROUND ART

OFDM (Orthogonal Frequency Division Multiplexing) is one type ofmulticarrier modulation method and is a method for performingcommunication by transmitting divided data on multiple subcarriers whichare orthogonal to each other. Each of the subcarriers is modulated byQPSK (Quadrature Phase Shift Keying), QAM (Quadrature AmplitudeModulation) or the like. When the subcarriers are orthogonal to eachother, each of the subcarriers has a frequency in which the power of theother subcarriers is set to zero as shown in FIG. 1. Thus, thesubcarriers can be densely arranged so that their waveforms areoverlapped with each other, allowing reduction in the frequency bands tobe used. Furthermore, the data is allocated to each of the subcarriers,and thus a data transfer rate in each of the subcarriers can be loweredcompared with the case of a single carrier. Consequently, a time, calleda guard interval, for absorbing a delayed wave can be provided for eachsymbol.

Moreover, in a multipath environment, a fading phenomenon that areception level of a certain frequency drops may occur. However, even ifthe data is partially lost, OFDM can reduce influences of the lost databy interleaving and error correction because the data is distributed tothe subcarriers in OFDM. OFDM is usually utilized in combination with amultiple access method such as FDMA (Frequency Division Multiple Access)and TDMA (Time Division Multiple Access).

Meanwhile, OFDMA (Orthogonal Frequency Division Multiple Access) is thesame as OFDM in dividing a carrier into multiple subcarriers as shown inFIG. 2. OFDMA is a method for achieving multiple access by sharing allthe subcarriers among all users, grouping a certain number ofsubcarriers as a subchannel, and adaptively allocating the subchannel toeach of the users at any timing.

Note that Japanese Patent Application Publication No. 2005-229468discloses a technique of receiving, from a communication partner device,a control signal used to provide a radio frequency band and atransmission frequency, and allowing a radio frequency band and atransmission frequency of an OFDM-modulated radio transmission signal tobe changed according to the control signal.

According to the above technique, for example, a control signalspecifying a radio frequency band and a transmission frequency for eachradio transmission signal is given to each of multiple cameras.Accordingly, each camera is allowed to change a frequency band or acenter frequency of the radio transmission signal according to the givencontrol signal. As a result, an output channel of a receiver forallocating a broadband channel and a narrowband channel can be changedas needed. Thus, it is possible to implement a wireless camera systemcapable of effectively utilizing limited frequency bands.

DISCLOSURE OF THE INVENTION

In a mobile communication system adopting OFDMA, ICI (Inter CarrierInterference) may be caused between adjacent subchannels by a frequencyoffset or jitter caused between a base station device and a mobilestation device. Specifically, ICI is less likely to occur if subcarriersof respective communication channels are not adjacent to each other, asshown in FIG. 3. On the other hand, when the subcarriers are adjacent toeach other as shown in FIG. 4, subcarrier orthogonality is lost due toICI. Thus, communication quality is deteriorated.

For example, as shown in FIG. 5, a mobile station device 14 b positionedat a cell edge of a cover area of a base station device 12 is morelikely to be influenced by ICI caused by frequency jitter due to its lowS/N (signal-to-noise) level. On the other hand, mobile station devices14 a and 14 c positioned near the base station device 12 are less likelyto be influenced by ICI because of their high S/N levels.

In order to prevent communication quality deterioration attributable toICI, it has heretofore been required to sacrifice any one of frequencyutilization efficiency and communication throughput. Specifically, whenpredetermined guard bands are always provided between adjacentcommunication channels to prevent occurrence of ICI, a data error rateis lowered. Accordingly, although the communication throughput isimproved, there is a problem that the frequency utilization efficiencyis lowered. Meanwhile, when data retransmission or error correction isperformed instead of providing the guard bands, the frequencyutilization efficiency is improved but a problem of lowering thecommunication throughput arises.

The present invention has been made in view of the conventional problemsdescribed above. It is an object of the present invention to provide amobile communication system, a base station device and a frequencyallocation method for the mobile communication system, which enableappropriate frequency allocation while improving both frequencyutilization efficiency and throughput in OFDMA radio communication.

In order to achieve the above object, a mobile communication systemaccording to the present invention includes multiple mobile stationdevices and a base station device for performing communication with eachof the mobile station devices by orthogonal frequency divisionmultiplexing in a predetermined frequency band. The base station deviceincludes: a frequency band storage unit configured to store multiplefrequency bands that can be respectively allocated to the mobile stationdevices, in association with predetermined conditions related tocommunication quality; a communication quality acquisition unitconfigured to acquire communication quality in communication with eachof the mobile station devices; and a frequency allocator configured toselect a frequency band to be allocated to each of the mobile stationdevices from the frequency band storage unit on the basis of thecommunication quality acquired by the communication quality acquisitionunit, and to notify the mobile station device of channel informationindicating the selected frequency band. Each of the mobile stationdevices communicates with the base station device in the frequency bandindicated by the channel information notified by the frequencyallocator.

A base station device according to the present invention performscommunications with multiple mobile station devices by orthogonalfrequency division multiplexing in predetermined frequency bands. Thebase station device includes: a frequency band storage unit configuredto store multiple frequency bands that can be respectively allocated tothe mobile station devices in association with predetermined conditionsrelated to communication quality; a communication quality acquisitionunit configured to acquire communication quality in communication witheach of the mobile station devices; and a frequency allocator configuredto select a frequency band to be allocated to each of the mobile stationdevices from the frequency band storage unit on the basis of thecommunication quality acquired by the communication quality acquisitionunit and to notify the mobile station device of channel informationindicating the selected frequency band.

Moreover, a frequency allocation method for a mobile communicationsystem according to the present invention is a frequency allocationmethod for a mobile communication system including multiple mobilestation devices and a base station device for performing communicationwith each of the mobile station devices by orthogonal frequency divisionmultiplexing in a predetermined frequency band. The method includes astep of storing, in the base station device, a plurality of frequencybands allocatable to each of the mobile station devices in a frequencyband storage unit, in association with predetermined conditions relatedto communication quality; a communication quality acquisition step ofacquiring, in the base station device, communication quality incommunication with each of the mobile station devices; a frequencyallocation step of selecting, in the base station device, a frequencyband to be allocated to each of the mobile station devices from thefrequency band storage unit on the basis of the communication qualityacquired in the communication quality acquisition step, and of notifyingthe mobile station device of channel information indicating the selectedfrequency band; and a step of communicating, in each of the mobilestation devices, with the base station device in the frequency bandindicated by the channel information notified in the frequencyallocation step.

According to the present invention, in the base station device, themultiple allocatable frequency bands are stored in association with thepredetermined conditions related to the communication quality. Then,based on communication quality in communication with each of the mobilestation devices, a frequency band to be allocated to the mobile stationdevice is selected. Moreover, channel information indicating theselected frequency band is notified to the mobile station device. Then,the mobile station device notified of the channel information by thebase station device communicates with the base station device in thefrequency band indicated by the channel information. According to thepresent invention, in OFDMA radio communication, multiple allocatablefrequency bands can be appropriately associated with the conditionsrelated to the communication quality. Thus, on the basis of thecommunication quality in communication with each of the mobile stationdevices, appropriate frequency allocation can be performed whileimproving both frequency utilization efficiency and throughput.

Moreover, according to one aspect of the present invention, a guard bandhaving a predetermined width according to the communication quality isprovided between the frequency bands stored in the frequency bandstorage unit. Thus, the number and bandwidth of the guard bands providedbetween the frequency bands can be reduced. Consequently, the frequencyutilization efficiency can be improved. Moreover, the frequencies in aportion where no guard bands are provided between the frequency bandscan be allocated to the mobile station device which performs highquality communication since ICI is unlikely to occur. Meanwhile, thefrequencies in a portion where the guard bands having a proper bandwidthaccording to the quality are provided can be allocated to the mobilestation device which performs low quality communication since ICI islikely to occur. Thus, the throughput can be improved while suppressinga data error rate.

Moreover, according to one aspect of the present invention, thecommunication quality acquisition unit acquires a difference inmagnitude between a desired signal level of a received signal and aninterference signal level in a frequency band allocated to each of themobile station devices. Thus, based on the difference in magnitudebetween the signal levels, the frequencies can be appropriatelyallocated to the respective mobile station devices.

Moreover, according to one aspect of the present invention, thecommunication quality acquisition unit acquires a frequency difference(frequency off set) between the frequency band allocated to the mobilestation device and the frequency band of the signal received from themobile station device. Thus, based on the magnitude of the frequencyoffset, the frequencies can be appropriately allocated to the respectivemobile station devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a frequency spectrum of OFDM.

FIG. 2 shows an example of subchannels in OFDMA.

FIG. 3 shows an example in which guard bands are provided between thesubchannels in OFDMA.

FIG. 4 shows an example in which interferences occur between adjacentsubchannels due to communication quality deterioration in OFDMA.

FIG. 5 is a configuration diagram of a mobile communication systemaccording to an embodiment of the present invention.

FIG. 6 is a functional block diagram of a base station device accordingto the embodiment of the present invention.

FIG. 7 shows an example of a correspondence relationship betweencommunication quality and allocated frequencies in a frequency bandstorage unit.

FIG. 8 shows an example of a correspondence relationship betweencommunication quality and allocated frequencies in the frequency bandstorage unit.

FIG. 9 is a functional block diagram of a mobile station deviceaccording to the embodiment of the present invention.

FIG. 10 is a functional block diagram of an AFC.

FIG. 11 is a flowchart illustrating frequency allocation processingbased on an S/N ratio.

FIG. 12 is a flowchart illustrating frequency allocation processingbased on a frequency offset.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to the drawings, an embodiment of the present inventionwill be described below. FIG. 5 shows a configuration of a mobilecommunication system 10 according to the embodiment of the presentinvention. The mobile communication system 10 includes at least one basestation device 12 and multiple mobile station devices 14.

FIG. 6 is a functional block diagram of the base station device 12. Asshown in FIG. 6, the base station device 12 includes a controller 20, aradio communication unit 30 and a line interface 40. The base stationdevice 12 transmits and receives radio signals by OFDMA to and from eachof the multiple mobile station devices 14, and also transmits andreceives packets to and from each of multiple other base station devicesthrough the line interface 40.

The radio communication unit 30 includes an antenna 31, a transmitter32, a receiver 33, an OFDM modulator 34, an OFDM demodulator 35, asignal processor 36 and a communication quality acquisition unit 37. Theantenna 31 is connected to the transmitter 32 and the receiver 33.

The receiver 33 includes a low noise amplifier, a down-converter and thelike. The receiver 33 down-converts an OFDM signal from the mobilestation device 14 into a base band OFDM signal, the OFDM signal beingreceived by the antenna 31, amplifies the base band OFDM signal andoutputs the signal to the OFDM demodulator 35 and the communicationquality acquisition unit 37. The OFDM demodulator 35 converts the baseband OFDM signal received from the receiver 33 into a digital signal,performs OFDM demodulation to extract subcarrier signals and outputseach of the extracted subcarrier signals to the signal processor 36 andthe communication quality acquisition unit 37.

The OFDM modulator 34 converts each subcarrier signal received from thesignal processor 36 into a base band OFDM signal by OFDM modulation, andoutputs the signal to the transmitter 32. The transmitter 32 includes anup-converter, a power amplifier and the like. The transmitter 32up-converts the base band OFDM signal received from the OFDM modulator34 into a radio frequency and outputs the radio frequency to the antenna31 after amplifying the radio frequency to a transmission output level.

The signal processor 36 includes a signal conversion circuit and thelike, and is connected to the OFDM modulator 34, the OFDM demodulator35, the communication quality acquisition unit 37 and the line interface40. The signal processor 36 separates and extracts data from each of thesubcarrier signals received from the OFDM demodulator 35 based oninformation related to communication quality received from thecommunication quality acquisition unit 37, and then outputs the data tothe line interface 40. Moreover, the signal processor 36 convertsmultiple pieces of data received from the line interface 40 intosubcarrier signals and outputs the signals to the OFDM modulator 34.

The communication quality acquisition unit 37 is connected to thereceiver 33, the OFDM demodulator 35, the signal processor 36 and afrequency allocator 24, and acquires communication quality incommunication with each of the mobile station devices. Specifically, thecommunication quality acquisition unit 37 acquires the communicationquality for each mobile station device based on received power valuesand the like of the subcarrier signal received from the OFDM demodulator35 and the OFDM signal received from the receiver 33. Specifically, thecommunication quality includes a transmission line distortion, an S/Nratio, a frequency offset and the like for each subcarrier in an uplinkto the base station device 12. Thereafter, the communication qualityacquisition unit 37 outputs information related to the communicationquality to the frequency allocator 24 and the signal processor 36.

The controller 20 includes a frequency band storage unit 22 and thefrequency allocator 24, and controls the entire base station device 12.Moreover, the controller 20 consists of a CPU, a memory and the like.

The frequency band storage unit 22 stores multiple frequency bands thatcan be allocated to the mobile station devices 14, respectively, inassociation with predetermined conditions related to the communicationquality. The predetermined conditions related to the communicationquality may be conditions related to a difference in magnitude between adesired signal level and an interference signal level, for example, anS/N ratio of a received signal. Alternatively, the predeterminedconditions related to the communication quality may be conditionsrelated to a frequency difference (frequency offset) from a frequencyband of a signal received from each of the mobile station devices 14.Moreover, guard bands, each having a predetermined width according tothe communication quality, may be provided between the frequency bandsstored in the frequency band storage unit 22.

FIG. 7 (a) shows an example of a correspondence relationship between thecommunication quality (S/N ratio) and the allocatable frequency bands,the correspondence relationship being stored in the frequency bandstorage unit 22. The frequency band storage unit 22 stores frequencybands related to multiple allocatable subchannels #1, #2, #3 . . . inassociation with predetermined conditions related to the S/N ratio.Specifically, in FIG. 7 (a), the subchannels of high frequencies arestored in association with the conditions that the S/N ratio is high,and the subchannels of low frequencies are stored in association withthe conditions that the S/N ratio is low.

Generally, the higher the S/N ratio, the less likely frequency jitteroccurs. Accordingly, ICI is less likely to occur. A guard band does notneed to be provided between the subchannels associated with the high S/Nratio, such as the subchannels #1 and #2 in FIG. 7 (a). Even if a guardband needs to be provided, it suffices that a guard band having a narrowbandwidth is provided. Meanwhile, when the S/N ratio is low, ICI islikely to be caused by frequency jitter. Thus, when the subchannelsassociated with the low S/N ratio are adjacent to each other, such asthe subchannels #2 and #3, and the subchannels #3 and #4 in FIG. 7 (a),guard bands having a sufficient bandwidth need to be provided inaccordance with the S/N ratio to prevent occurrence of ICI.

FIG. 7 (b) shows an example of a correspondence relationship between theS/N ratio and the allocatable frequency bands, the correspondencerelationship being stored in the frequency band storage unit 22, as inthe case of FIG. 7 (a). Unlike FIG. 7 (a), the frequency band storageunit 22 stores subchannels near the center of the frequency band inassociation with conditions that the S/N ratio is high, and storessubchannels near edges of the frequency band in association withconditions that the S/N ratio is low. A guard band does not need to beprovided between the subchannels associated with the high S/N ratio,such as the subchannels #2 and #3 in FIG. 7 (b). Meanwhile, when thesubchannels associated with the low S/N ratio are adjacent to eachother, such as the subchannels #1 and #2, and the subchannels #3 and #4,guard bands having a sufficient bandwidth need to be provided inaccordance with the S/N ratio.

The multiple allocatable frequency bands are appropriately stored inassociation with the conditions related to the communication qualitysuch as the S/N ratio. Thus, the number and bandwidth of the guard bandsprovided between the subchannels for each of the frequency bands can bereduced. Consequently, frequency utilization efficiency can be improved.Moreover, the subchannels in a portion where no guard bands are providedor the guard bands having a narrow bandwidth are provided can beallocated to the mobile station device in which the S/N ratio is high sothat ICI is less likely to occur. Meanwhile, the frequencies in aportion where the guard bands having a sufficient bandwidth according tothe S/N ratio are provided can be allocated to the mobile station devicein which the S/N ratio is low so that ICI is more likely to occur. Thus,throughput can be improved while suppressing a data error rate.

Meanwhile, FIG. 8 (a) shows an example of a correspondence relationshipbetween the communication quality (frequency offset) and the allocatablefrequency bands, the correspondence relationship being stored in thefrequency band storage unit 22. The frequency band storage unit 22stores frequency bands related to multiple allocatable subchannels 141,#2, #3 . . . in association with predetermined conditions related to thefrequency offset. Specifically, in FIG. 8 (a), the subchannels of highfrequencies are stored in association with the conditions that thefrequency offset is small, and the subchannels of low frequencies arestored in association with the conditions that the frequency offset islarge.

Generally, the smaller the frequency offset, the more easily thesubcarrier orthogonality is maintained. Accordingly, ICI is less likelyto occur. A guard band does not need to be provided between thesubchannels associated with the small frequency offset, such as thesubchannels #1 and #2 in FIG. 8 (a). Even if a guard band needs to beprovided, it suffices that a guard band having a narrow width isprovided. Meanwhile, when the frequency offset is large, the subcarrierorthogonality is easily lost. Accordingly, ICI is more likely to occur.Thus, when the subchannels associated with the large frequency offsetare adjacent to each other, such as the subchannels #2 and #3, and thesubchannels #3 and #4 in FIG. 8 (a), guard bands having a sufficientbandwidth need to be provided in accordance with the frequency offset toprevent occurrence of ICI.

FIG. 8 (b) shows an example of a correspondence relationship between thefrequency offset and the allocatable frequency bands, the correspondencerelationship being stored in the frequency band storage unit 22, as inthe case of FIG. 8 (a). Unlike FIG. 8 (a), the frequency band storageunit 22 stores subchannels near the center of the frequency band inassociation with conditions that the frequency offset is small, andstores subchannels near edges of the frequency band in association withconditions that the frequency offset is large. A guard band does notneed to be provided between the subchannels associated with the smallfrequency offset, such as the subchannels #2 and #3 in FIG. 8 (b).Meanwhile, when the subchannels associated with the large frequencyoffset are adjacent to each other, such as the subchannels #1 and #2,and the subchannels #3 and #4, guard bands having a sufficient bandwidthneed to be provided in accordance with the frequency offset.

The multiple allocatable frequency bands are appropriately stored inassociation with the conditions related to the communication qualitysuch as the frequency offset. Thus, the number and bandwidth of theguard bands provided between the subchannels for each of the frequencybands can be reduced. Consequently, frequency utilization efficiency canbe improved. Moreover, the subchannels in a portion where no guard bandsare provided or the guard bands having a narrow bandwidth are providedcan be allocated to the mobile station device in which the frequencyoffset is small so that ICI is less likely to occur. Meanwhile, thefrequencies in a portion where the guard bands having a sufficientbandwidth according to the frequency offset are provided can beallocated to the mobile station device in which the frequency offset islarge so that ICI is more likely to occur. Thus, throughput can beimproved while suppressing a data error rate.

The frequency allocator 24 is connected to the communication qualityacquisition unit 37 and the frequency band storage unit 22. Thefrequency allocator 24 selects the frequency band to be allocated toeach of the mobile station devices 14 from the frequency band storageunit 22, on the basis of the communication quality in communication witheach of the mobile station devices 14, the communication quality beingreceived from the communication quality acquisition unit 37. Thereafter,the frequency allocator 24 notifies each of the mobile station devices14 of channel information indicating the selected frequency band. Notethat the communication quality received from the communication qualityacquisition unit 37 includes, as described above, the transmission linedistortion, the S/N ratio, the frequency offset and the like for eachsubcarrier in the uplink to the base station device 12. Moreover, thechannel information to be notified to each of the mobile station devices14 may include the selected frequency band or each subcarrier frequencywithin the subchannel, or may include identification information forspecifying the selected frequency band or the subchannel.

Here, with reference to FIG. 5 and FIG. 7 (a), a description will begiven of an exemplar processing by the frequency allocator 24 forselecting the frequency to be allocated to each of the mobile stationdevices 14 based on the S/N ratio. FIG. 5 shows an example where mobilestation devices 14 a and 14 c are located near the base station device12 and a mobile station device 14 b is located near a cell edge.Moreover, the frequency band storage unit 22 shown in FIG. 7 (a) storessubchannels #1 and #2 of high frequencies in association with conditionsthat the S/N ratio is high, and stores subchannels #3 and #4 of lowfrequencies in association with conditions that the S/N ratio is low. Inthis case, the communication quality acquisition unit 37 acquires a highS/N ratio for the mobile station devices 14 a and 14 c, and acquires alow S/N ratio for the mobile station device 14 b. Thereafter, thefrequency allocator 24 selects a frequency band related to thesubchannels #1 and #2 associated with the high S/N ratio from thefrequency band storage unit 22 shown in FIG. 7 (a), and notifies themobile station devices 14 a and 14 c of channel information (thesubchannels #1 and #2) indicating the frequency band. Meanwhile, thefrequency allocator 24 selects a frequency band related to thesubchannel #4 associated with the low S/N ratio for the mobile stationdevice 14 b, and notifies the mobile station device 14 b of channelinformation (the subchannel #4) indicating the frequency band.

As described above, the subchannels #1 and #2 in a portion where noguard bands are provided or the guard bands having a narrow bandwidthare provided are allocated to the mobile station devices 14 a and 14 cin which the S/N ratio is high so that ICI is less likely to occur.Meanwhile, the subchannel #4 in a portion where the guard band having asufficient bandwidth is provided is allocated to the mobile stationdevice 14 b in which the S/N ratio is low so that ICI is more likely tooccur. Note that the above processing is executed when the mobilestation device 14 establishes a link to the base station device 12 or atan arbitrary timing during communication.

FIG. 9 is a functional block diagram of the mobile station device 14. Asshown in FIG. 9, the mobile station device 14 includes a controller 50,a radio communication unit 60, a storage unit 80, an operation unit 82and a display unit 84. The mobile station device 14 transmits andreceives radio signals by OFDMA to and from the base station device 12and communicates with the base station device in the frequency bandindicated by the channel information notified by the frequency allocator24 of the base station device 12.

The storage unit 80 operates as a work memory of the controller 50.Moreover, the storage unit 80 holds programs, parameters and the likerelated to various processing to be executed by the controller 50. Theoperation unit 82 is, for example, a ten key pad or the like forreceiving input of a telephone number or a character string by a userand outputting the received input to the controller 50. The display unit84 is formed of a liquid crystal display unit, for example, and displaysinformation such as characters and images according to signals receivedfrom the controller 50. The controller 50 includes a CPU, a memory andthe like, and controls the entire mobile station device 14.

The radio communication unit 60 includes an antenna 61, a transmitter62, a receiver 63, an OFDM modulator 64, an OFDM demodulator 65, asignal processor 66 and an AFC 70 (Automatic Frequency Control). Theantenna 61 is connected to the transmitter 62 and the receiver 63.

The receiver 63 includes a low noise amplifier, a down-converter and thelike. The receiver 63 down-converts an OFDM signal received from thebase station device 12 through the antenna 61 into a base band OFDMsignal. Thereafter, the receiver 63 amplifies the base band OFDM signaland outputs the signal to the OFDM demodulator 65. The receiver 63 alsodetects a symbol timing from the base band OFDM signal and outputs, tothe AFC 70, a signal having a frequency corresponding to the symboltiming as a reference signal. The OFDM demodulator 65 converts the baseband OFDM signal received from the receiver 63 into a digital signal,performs OFDM demodulation to extract subcarrier signals, and outputseach of the extracted subcarrier signals to the signal processor 36.

The AFC 70 is connected to the receiver 63 and the OFDM modulator 64,and outputs, on the basis of the reference signal received from thereceiver 63, a signal (hereinafter referred to as a symbol frequencysignal) having a frequency synchronized with the symbol timing of thebase band OFDM signal to the OFDM modulator 64. FIG. 10 is a functionalblock diagram of the AFC 70. As shown in FIG. 10, the AFC 70 includes aphase comparator 72, a loop filter 74 (integrating circuit/low-passfilter) and a voltage controlled oscillator 76 (VCO), and can output afrequency signal fully synchronized with a reference signal inputtedfrom the outside. The phase comparator 72 detects a phase differencebetween the reference signal inputted from the outside and a frequencysignal fed back from the voltage controlled oscillator 76. Thereafter,the phase comparator 72 outputs, to the loop filter 74, the phasedifference component as a phase difference signal on a pulse. The loopfilter 74 outputs, to the voltage controlled oscillator 76, a signalconverted into a direct current by blocking a high frequency componentof the phase difference signal received from the phase comparator 72.The voltage controlled oscillator 76 is an oscillator configured tochange an oscillating frequency according to a voltage applied to acontrol terminal. The voltage controlled oscillator 76 adjusts theoscillating frequency according to a control voltage applied by the loopfilter 74, and outputs a frequency signal having a frequency fullysynchronized with the reference signal inputted to the AFC 70.

The OFDM modulator 64 is connected to the transmitter 62, the signalprocessor 66 and the AFC 70. The OFDM modulator 64 converts thesubcarrier signal received from the signal processor 66 into a base bandOFDM signal by OFDM modulation based on the symbol frequency signalreceived from the AFC 70, and outputs the base band OFDM signal to thetransmitter 62. Accordingly, a symbol timing of the OFDM signal to betransmitted by the mobile station device 14 is synchronized with thesymbol timing of the OFDM signal received from the base station device12. Thus, a frequency drift between the mobile station device 14 and thebase station device 12 is eliminated. The transmitter 62 includes anup-converter, a power amplifier and the like. The transmitter 62up-converts the base band OFDM signal received from the OFDM modulator64 into a radio frequency and outputs the radio frequency to the antenna61 after amplifying the radio frequency to a transmission output level.

Note that, when the channel information is notified from the basestation device 12, in other words, when a new frequency band to be usedfor communication with the base station device 12 is allocated, themobile station device 14 determines a subcarrier frequency to be usedfor subsequent communication on the basis of the channel information.Thus, the mobile station device 14 can communicate with the base stationdevice 12 in the frequency band newly allocated by the base stationdevice 12.

Next, with reference to a flowchart of FIG. 11, a description will begiven of processing of allocating a frequency to the mobile stationdevice 14 based on the S/N ratio. This processing is executed when themobile station device 14 establishes a link to the base station device12 or at an arbitrary timing during communication with the mobilestation device 14. The frequency band storage unit 22 previously storesmultiple frequency bands that can be respectively allocated to themobile station devices 14 in association with predetermined conditionsrelated to the S/N ratio.

When this processing is started in the base station device 12, thereceiver 33 outputs, to the communication quality acquisition unit 37,an OFDM signal received from the mobile station device 14 through theantenna 31 in 8100.

Next, the communication quality acquisition unit 37 acquires an S/Nratio of the mobile station device 14 based on a received power valueand the like of the OFDM signal received from the receiver 33 (S102),and outputs the S/N ratio to the frequency allocator 24. The frequencyallocator 24 selects a frequency band to be allocated to the mobilestation device 14 from the frequency band storage unit 22 based on theS/N ratio of the mobile station device 14, the S/N ratio being receivedfrom the communication quality acquisition unit 37 (S104).

Thereafter, the base station device 12 notifies the mobile stationdevice 14 of channel information (for example, information includingsubchannel numbers) indicating the selected frequency band (S110). Themobile station device 14 notified of the channel information by the basestation device 12 performs subsequent communication with the basestation device 12 in the frequency band indicated by the channelinformation.

Meanwhile, FIG. 12 illustrates processing of allocating a frequency tothe mobile station device 14 based on a frequency offset. Hereinafter,the same steps as those described with reference to FIG. 11 are denotedby the same reference numerals, and repetitive description will beomitted. The frequency band storage unit 22 previously stores multiplefrequency bands that can be respectively allocated to the mobile stationdevices 14 in association with predetermined conditions related to thefrequency offset.

The communication quality acquisition unit 37 acquires a frequencyoffset of the mobile station device 14 based on a received power valueand the like of the OFDM signal received from the receiver 33 (S106),and outputs the frequency offset to the frequency allocator 24. Thefrequency allocator 24 selects a frequency band to be allocated to themobile station device 14 from the frequency band storage unit 22 basedon the frequency offset of the mobile station device 14, the frequencyoffset being received from the communication quality acquisition unit 37(S108).

The mobile communication system described above enables appropriatefrequency allocation while improving both frequency utilizationefficiency and throughput in OFDMA radio communication.

Note that the present invention is not limited to the above embodiment.For example, the present invention is applicable not only to a mobilecommunication system but also to various OFDMA radio communicationsystems including a base station device and multiple terminal devices.

Moreover, the entire contents of Japanese Patent Application No.2006-124195 (filed on Apr. 27, 2006) are incorporated herein byreference.

INDUSTRIAL APPLICABILITY

As described above, the mobile communication system, the base stationdevice and the frequency allocation method for the mobile communicationsystem according to the present invention can realize appropriatefrequency allocation while improving both frequency utilizationefficiency and throughput in OFDMA radio communication. Thus, the mobilecommunication system, the base station device and the frequencyallocation method for the mobile communication system are advantageousin radio communication such as mobile communication.

1. A mobile communication system comprising: a plurality of mobilestation devices; and a base station device for performing communicationwith each of the mobile station devices by orthogonal frequency divisionmultiplexing in a predetermined frequency band, wherein the base stationdevice includes: a frequency band storage unit configured to store aplurality of frequency bands allocatable to each of the mobile stationdevices, in association with predetermined conditions related tocommunication quality; a communication quality acquisition unitconfigured to acquire communication quality in communication with eachof the mobile station devices; and a frequency allocator configured toselect a frequency band to be allocated to each of the mobile stationdevices from the frequency band storage unit on the basis of thecommunication quality acquired by the communication quality acquisitionunit, and to notify the mobile station device of channel informationindicating the selected frequency band, and each of the mobile stationdevices communicates with the base station device in the frequency bandindicated by the channel information notified by the frequencyallocator.
 2. The mobile communication system according to claim 1,wherein a guard band having a predetermined width according to thecommunication quality is provided between the frequency bands stored inthe frequency band storage unit.
 3. The mobile communication systemaccording to any one of claim 1 and 2, wherein the communication qualityacquisition unit acquires a difference in magnitude between a desiredsignal level and an interference signal level according to a receivedsignal in a frequency band allocated to each of the mobile stationdevices.
 4. The mobile communication system according to any one ofclaim 1 and 2, wherein the communication quality acquisition unitacquires a frequency difference between a frequency band allocated toeach of the mobile station devices and a frequency band of a signalreceived from the mobile station device.
 5. A base station device forperforming communication with a plurality of mobile station devices byorthogonal frequency division multiplexing in predetermined frequencybands, the base station device comprising: a frequency band storage unitconfigured to store a plurality of frequency bands allocatable to eachof the mobile station devices, in association with predeterminedconditions related to communication quality; a communication qualityacquisition unit configured to acquire communication quality incommunication with each of the mobile station devices; and a frequencyallocator configured to select a frequency band to be allocated to eachof the mobile station devices from the frequency band storage unit onthe basis of the communication quality acquired by the communicationquality acquisition unit, and to notify the mobile station device ofchannel information indicating the selected frequency band.
 6. Afrequency allocation method for a mobile communication system includinga plurality of mobile station devices and a base station device forperforming communication with each of the mobile station devices byorthogonal frequency division multiplexing in a predetermined frequencyband, the frequency allocation method comprising: to a step of storing,in the base station device, a plurality of frequency bands allocatableto each of the mobile station devices in a frequency band storage unit,in association with predetermined conditions related to communicationquality; a communication quality acquisition step of acquiring, in thebase station device, communication quality in communication with each ofthe mobile station devices; a frequency allocation step of selecting, inthe base station device, a frequency band to be allocated to each of themobile station devices from the frequency band storage unit on the basisof the communication quality acquired in the communication qualityacquisition step, and of notifying the mobile station device of channelinformation indicating the selected frequency band; and a step ofcommunicating, in each of the mobile station devices, with the basestation device in the frequency band indicated by the channelinformation notified in the frequency allocation step.